The presence of an Al2O3 inclusion in an Al or Al alloy sputter target can result in arcing when the target is sputtered in a sputtering apparatus. During sputtering, an electric field forms in the sputtering apparatus between the target and an anode. This electric field ionizes a gas, such as argon, within the sputtering apparatus so as to form a plasma. Typically, a plasma sheath, or dark space, separates a positive column of the plasma from the sputter target. This sheath has a certain thickness. Introduction of an Al2O3 inclusion on the surface of the target can distort the electric field so as to alter the shapes of the positive plasma column and the plasma sheath.
Over time, electrical charges can build up in the vicinity of an Al2O3 inclusion. When the electrical charge imbalance becomes sufficiently strong, a high current density cathodic arc forms. The high current density cathodic arc heats a small section of the target surface, often sufficiently to melt the target material in that section. The arc pressure causes droplets of liquid metal to eject from the sputtering target surface at high velocity and strike an intended substrate, such as a silicon chip. The droplets, or macroparticles, solidify on the substrate creating large defects thereon. These macroparticles can range in size from less than 1 μm to greater than 50 μm in diameter and can reduce significantly device yields, for example, in integrated circuit manufacturing.
Dielectric inclusions and surface layers have long been known to cause arcing in plasma discharges as well as in vacuum spark gaps. More recently, research on arcing in sputtering plasmas has shown that inclusion and surface oxide induced arcing causes molten metal macroparticle ejection from aluminum sputtering targets producing particle defects on the substrate. High-speed video analysis of arcing from heavily doped aluminum-aluminum oxide sputtering targets has shown that the molten metal macroparticles ejected therefrom can have speeds of over 500 m/sec and temperatures of 3000 K.
It has been reported that dielectric inclusions between 0.10 and 10 μm cause arc initiation in a hydrogen plasma with 0.1 mA/cm2 discharge current; hydrogen pressures between 2.7 and 13 Pa; and a cathode bias between 100 and 500 volts. Also, arcing from aluminum targets sputtered in 1014 ions/cm3 argon, hydrogen and nitrogen plasmas with 1-μm diameter Al2O3 inclusions on the cathode surface has been reported. Finally, evidence for a critical size effect for arc initiation from inclusions in hydrogen tokomak plasmas has been reported but critical inclusion sizes were not measured.
Notably, attempts have been made to reproduce the above results where inclusion sizes were measured. As a result, it was determined that the arcs were a result of surface contamination and not the size of the inclusions. Apparently, the small values for the critical size of the inclusion for arc initiation that initially were reported appeared to be due to surface contamination effects. As such, it is important to provide contaminant-free sputter targets when examining the effect of inclusion sizes on arc initiation.
Consequently, there remains a need in the art for methods to inhibit bipolar arcing in Al or Al alloy sputter targets having Al2O3 inclusions. Such methods are calculated to improve device yields and decrease scrap, thereby reducing manufacturing costs in fields such as the manufacture of integrated circuits.